The present invention relates to a linear amplifier for use mainly in the high frequency band and, more particularly, to a feed-forward amplifier which includes an error detection circuit which detects a nonlinear distortion component of a main amplifier and an error rejection circuit which amplifies the detected distortion component by use of an auxiliary amplifier and then injects it into the output of the main amplifier to thereby cancel the error component.
FIG. 1 shows the basic construction of a conventional feed-forward amplifier. The feed-forward amplifier is comprised basically of two signal cancellation circuits, i.e. an error detection circuit 1 and an error rejection circuit 2. The error detection circuit 1 includes a signal amplification path 3 and a linear signal path 4, and the error rejection circuit 2 includes a linear signal path 5 and an error injection path 6. The signal amplification path 3 is formed by a cascade connection of a main amplifier 7, a variable attenuator 8 and a variable delay line or phase shifter 9, whereas the linear signal path 4 is formed by a transmission line. The linear signal path 5 is formed by a transmission line, whereas the error injection path 6 is formed by a cascade connection of a variable attenuator 10, a variable delay line 11 and an auxiliary amplifier 12. Even if both or either one of the variable attenuator 8 and the variable delay line 9 is provided in the linear signal path 4, there would be no appreciable characteristic difference. Similarly, both or either one of the variable attenuator 10 and the variable delay line 11 may also be included in the linear signal path 5. A power divider 13, a power combiner/divider 14 and a power combiner 15 are each a simple loss-free power divider/combiner composed of a transformer or a hybrid circuit. A description will be given of the operation of the feed-forward amplifier.
An input signal to an input terminal 16 is divided first by the power divider 13 to the paths 3 and 4 and then the divided signals are combined by the power combiner/divider 14. The variable attenuator 8 and the variable delay line 9 are adjusted so that the two signal components divided from the respective paths 3 and 4 to the error injection path 6 via the power combiner/divider 14 are equal in amplitude and delay but anti-phase relative to each other. In this instance, the condition for the anti-phase relationship can be implemented by a proper selection of the phase shift amount between input and output ports of the power divider 13 or power combiner/divider 14, or by a phase inversion in the main amplifier 7, or by inserting a phase inversion circuit having a short-circuit termination at one terminal of a circulator 18, such as depicted in FIG. 2, in either one of the paths 3 and 4. Since the error detection circuit 1 is constructed as mentioned above, a difference component between the two signals on the two paths 3 and 4 is detected as the output from the power combiner/divider 14 to the path 6. This difference component is exactly the error comprised of signal distortion and noise which are produced by the main amplifier 7 itself; therefore, the circuit 1 is called an error detection circuit.
The variable attenuator 10 and the variable delay line 11 are adjusted so that transfer functions of the two paths 5 and 6 from an input port 14a of the power combiner/divider 14, which is the output terminal of the path 3, to an output terminal 17 of the power combiner 15 are equal in terms of amplitude and delay but bear an anti-phase relationship to each other. Since the input signal to the path 6 is the error component of the main amplifier 7 detected by the error detection circuit 1, the path 6 injects the error component into the output signal of the main amplifier 7 in anti-phase and equal amplitude relationships thereto at the output terminal 17 of the power combiner 15, and as a result of this, the error component in the output of the entire feed-forward amplifier circuit is cancelled.
The above is the operation of an ideal feed-forward amplifier, but it is difficult, in practice, to completely adjust the two paths in each of the error detection circuit 1 and the error rejection circuit 2 so that they bear the above-mentioned anti-phase and equal amplitude relationship to each other. Even if the initial adjustment were perfect, characteristics of the amplifiers used would vary with variations in the ambient temperature, the power supply voltage, etc.; so that it is extremely difficult to maintain the equilibrium of the two paths for a long period of time. FIG. 3 is a graph showing the relationship between deviations of the amplitude and phase of two signal components on the two paths of each of the circuits 1 and 2 from their equal amplitude and anti-phase requirements and the amount of signal suppression. It is seen from FIG. 3 that deviations of the phase and amplitude need to be within .+-.1.8 degrees and within .+-.0.3 dB, respectively, for example, and that severe limitations are imposed on the equilibrium of transmission characteristics of the two paths and completeness of their adjustment therefor. When the equilibrium or balance of the error detection circuit 1 is lost by a change in the ambient temperature, a voltage fluctuation of the power supply, or long term fluctuation of characteristics of circuit elements, a component of the main signal (i.e. the input signal component to the input terminal 16) is added to the input to the auxiliary amplifier 12 at a level higher than the error component, generating an unnecessary distortion. When the balance of the error rejection circuit 2 is lost, the amount of signal suppression is reduced and the amount of distortion improvement as by the feed-forward amplifier is deteriorated accordingly.
U.S. Pat. No. 4,580,105 discloses an arrangement in which a pilot signal is injected via a coupler into the signal amplification path 3 of the main amplifier 7 at the input side thereof in the feed-forward amplifier of FIG. 1 and the variable attenuator 10 and the variable delay line 11 are automatically controlled in such a manner as to minimize the level of the pilot signal component which is detected in the output of the feed-forward amplifier. With such automatic control, it is possible to retain the equilibrium in the error rejection circuit 2 but impossible to maintain the equilibrium in the error detection circuit 1. Consequently, the equilibrium of the error detection circuit 1 is destroyed with the lapse of time, resulting in the amount of distortion improvement as by the feed-forward amplifier being reduced correspondingly.
Japanese Patent Application Laid Open No. 198,809/1989 discloses an arrangement in which a pilot signal is injected via a coupler into the signal input path between the input terminal 16 and the power divider 13 in the feed-forward amplifier of FIG. 1, and the variable attenuator 8 and the variable delay line 9 of the error detection circuit 1 are automatically controlled in such a manner as to minimize the level of the pilot signal component which appears in the output from the auxiliary amplifier 12 of the error rejection circuit 2. In the case where the pilot signal is injected into the signal amplification path 3 as set forth in the above-noted U.S. patent, substantially no pilot signal component is provided at the output terminal 17 of the feed-forward amplifier, whereas when the pilot signal is injected into the input side of the feed-forward amplifier as described in the above-mentioned Japanese application, an amplified pilot signal is provided at the output terminal 17. Accordingly, during control of the balance of the error detection circuit 1 the feed-forward amplifier cannot be used for signal amplification.